Lithium ion secondary batteries, a typical representative of non-aqueous electrolyte secondary batteries, have a high electromotive force and a high energy density, and therefore a demand is increasing for lithium ion secondary batteries as a main power source for mobile telecommunication devices and portable electronic devices. Majority of the lithium ion secondary batteries currently on the market use a lithium composite oxide mainly composed of cobalt as the positive electrode active material. However, in the case of the lithium composite oxide mainly composed of cobalt, because the cost of the cobalt compound used as the raw material is high, there have been vigorous researches also on the lithium composite oxide composed mainly of nickel.
Upon charging, a lithium composite oxide mainly composed of Co or Ni contains Co4+ or Ni4+ that are highly reactive and high in valence. Due to this fact, under a high-temperature environment, the electrolyte decomposition reaction involving the lithium composite oxide is accelerated to generate gas in the battery, making it difficult to curb the heat generation at the time of shorting.
The following may be the reasons why it is difficult to curb the heat generation at the time of shorting. When shorting is caused for example by a nail penetration, Joule heat is generated at the short circuit portion. The heat induces the thermal decomposition reaction of the positive electrode active material and the reaction between the active material and the electrolyte. Since these reactions involve heat generation, when the reactions cannot be curbed, abnormal heat is generated in the battery.
The thermal decomposition reaction of the active material is the reaction of oxygen desorption from the active material surface, and the electrolyte decomposition reaction is a reaction between the active material surface and the electrolyte. As a result of various examinations, it was found that these reactions promote at active sites of the active material surface that are formed due to lattice defects.
Thus, to secure the safety at the time of shorting, there has been proposed that a predetermined metal oxide coating film is formed on the active material surface (patent documents 1 to 7).
On the other hand, to secure the safety upon overcharging, there have been proposed a mechanism for mechanically shutting down the current by using an increase in the battery internal pressure, a mechanism for shutting down the current with a PTC element by using an increase in battery temperature, and a mechanism for shutting down the current with a shutdown function of the separator of polyolefin having a low melting point. Also proposed is a method in which a starting material of the conductive polymer which polymerizes upon overcharging is added to the electrolyte, to create a minute short circuit portion with the conductive polymer upon overcharging in the battery to allow an automatic discharge (hereinafter, referred to as internal short-circuit safety mechanism). (Patent Document 8)    [Patent Document 1] Japanese Laid-Open Patent Publication No. Hei 8-236114    [Patent Document 2] Japanese Laid-Open Patent Publication No. Hei 9-35715    [Patent Document 3] Japanese Laid-Open Patent Publication No. Hei 11-317230    [Patent Document 4] Japanese Laid-Open Patent Publication No. Hei 11-16566    [Patent Document 5] Japanese Laid-Open Patent Publication No. 2001-196063    [Patent Document 6] Japanese Laid-Open Patent Publication No. 2003-173775    [Patent Document 7] Japanese Unexamined Patent Application No. 2003-500318    [Patent Document 8] Japanese Laid-Open Patent Publication No. Hei 10-321258